Optical parametric amplifier, oscillator, and limiter using non-phase matchable interaction



Aug. 16, 1966 A. AsHKlN 3,267,335

OPTICAL PRAMETRIC AMPLIFIER, OSCILLATOR, AND

LIMITER USING NON-PHASE MATCHABLE INTERACTION Filed Aug. 19, 1965 3Sheets-Sheet l Bywwww ATTORNEY Aug. 16, 1966 A. AsHKlN 3,267,385

OPTICAL PARAMETRIC AMPLIFIER, OSCILLATOR, AND LIMITEE USING NON-PHASEMATCHABLE INTERACTION Filed Aug. 19, 1965 3 Sheets-Sheet 2 FIG. 3

PULSE@ LASER C 3,267,385 Ice Patented August 16, 1966 corporation of NewYork Filed Aug. 19, 1965, Ser. No. 480,986 11 Claims. (Cl. 33o- 4.6)

This invention relates to the generation and amplification ofelectromagnetic waves having wavelengths cornparable to or shorter thanlthe dimensions lof the amplifying device.

rIt Will be convenient hereinafter to discuss the invention withparticular reference to light waves and waves of shorter wavelength; butit is to be understood that the principles are similarly applicable tolonger wavelengths, such as wavelengths in the microwave range.

The recently developed optical parametric amplifiers have complementedthe more highly publicized laser amplifiers `by providing amplificationbandwidths that are many times broader than laser bandwidths. In someproposed optical communication systems, particularly multiple channelsystems, it is desirable to provide broadband 'amplifiers for repeatingthe signal-modulated beam.

An optical parametric amplifier typically comprises a material that hasa nonlinear, i.e., amplitude-dependent, response to each incident lightWave. In addition to an information-modulated wave -to be amplified, aso-called pumping wave of .another frequency is applied to the materialin such a way as to interact with and transfer energy to theinformation-modulated wave. The amplification of theinformation-modulated Wave, as produced by this transfer of energy, iscalled parametric gain.

lIt has been 'heretofore recognized that traveling wave interactions areadvantageous for efficient parametric devices. A traveling waveinteraction occurs when the transfer of energy from the pumping wave tothe information-modulated wave Ican be maintained over .as large avolume of the nonlinear medium as the two vvaves can trave-rse incommon, regardless of the number of wavelengths of each wave that areinvolved. Such traveling wave interactions are to vbe contrasted withinteractions providing transfer of energy in the desired sense only overla restricted length :and providing transfer of energy in the reversesense between the Waves for any further common propagation of the waves.The main line of development, both of optical second harmonic generatorsand of optical parametric amplifiers, has centered around the use ofbitefringent materials to obtain phase-matching, which enables travelingwave parametric amplification. Phase matching is the process of makingthe sum of the signal and idler phase vectors equal tothe pumping phasevector. A phase vector is related to the product of index of refractionand frequency, or to index divided by wavelength. Birelringence has beenthought to be necessary for phase matching, because the indices ofrefraction of nearly all materials vary with frequency.

The apparent limitation of optical traveling wave .parametric amplifiersto optically fbirefringent materials has had the unfortunate `effectthat such nighly nonlinear lighttransmitting materials as galliumarsenide (GaAs) have 'been neglected in the earch for better parametricamplifiers because they are not bire-fringent. Moreover, other nonlinearmaterials that are birefringent, such as lithium niobate (LiNbOa), havesome very strong nonlinearities that are commonly callednon-phase-matchable, i.e., for which the phase relationships have beenconsidered to be inappropriate for traveling wave interactions. Forexample, all of the waves coupled through such .a nonlinearity may havethe same polarization, so that the bircfrmgence cannot lbe used forphase matching. Such interact1ons in either type of material Will bereferred to herematter as non-phase-matchable, for the sake of brevity.Of course, other weaker nonlinearities in a birefringent material may bephase-matchable; lithium niobate has both types of nonlinearities.

While it has been recognized heretofore that small deviations from adesired phaseamatching condition can be compensated if phase-matching isinherently possi-ble, it has not been considered possible to compensatethe substantial irreducible phase mismatch occurring in nonlinearinteractions wh-en the medium is not birefringent or when thepolarization of the interacting waves are alike.

An object of my invention is traveling wave parametric amplificationthrough nonlinear optical interactions that are non-phase-matchable.

My invention resides in my recognition that substantial and irreduciblevector phase mismatches in the nonlinear coupling of optical signal andpumping Waves can |be compensated 'by providing .a sufficiently highpumping .power level in relation to other relevant parameters.

Further, my invention resides in my recognition that laser emissions canbe produced with lsufiicient polwer and spectral purity to provide sucha pumping beam.

An advantage of my invention is that parametric gain in materials suchas `galliurn arsenide and lithium niobate lcan greatly exceed thatachieved heretofore in parametric interactions.

A separate feature of my invention resides in the employment of curvedfocusing surfaces upon the nonlinear optical medium in order to reducethe pumping beam radius therein, thereby reducing the required pumpmgpower.

A more complete understanding of my invention will be obtained from thefollowing detailed description in conjunction with the drawing, inwhich:

FIG. 1 is a partially block diagrammatic and partially pictorial showingof a single channel parametric amplifier according to the invention;

Y FIG. 2 is a partially block diagrammatic and partially pictorial:showing of a multiple channel parametric amplifier 'according to theinvention;

p FIG. `3 is a partially block diagrammatic and partially pictorialshowing of Ian optical power limiter according to the invention; |andFIG. 4 is a partially block diagrammatic and partially pictorial showingof a multiple frequency oscillator according to the invention.

In order to understand the invention more fully, it is desirable firstto understand the nature of second harmonic generation and opticalparametric amplification.

In second harmonic generation the driving, or fundamental, beam inducesan electric polarization wave in the material, i.e., essentially awave-like motion of electric dipoles in the material. In anoncentrosymmetric, i.e., piezoelectric, material the electricpolarization is not proportional to the electric field strength of thedriving wave;

and, indeed, it has a significant component that is proportional to thesquare `of the electric field strength of the driving wave. Thiscomponent of the electric polarization is not sensitive to the polarityof the electric field intensity of the driving Wave and varies at twicethe frequency of the driving wave. The moving charges radiate energy attwice the frequency of the driving wave.

It is characteristic of second harmonic generation that the phase of theinduced electric polarization wave is locked to the phase of the drivingwave, which is the only applied wave. Unless phase matching conditionsare satisfied for the driving wave and the radiated second harmonicwave, for example, as taught in the copending application of J. A.Giordmaine et al. Serial No. 158,267, filed December l1, 1961, nowPatent No. 3,234,475 the second harmonic waves radiated from variouspoints within the crystal do not interfere constructively outside of thematerial. That is, no effective traveling wave generation of the secondharmonic is obtained. Variation of the driving power does not affect therelative phases of fundamental and harmonic waves when phase-matchingconditions are not satisfied.

On the other hand, in parametric amplification, at least two appliedbeams of different frequencies, the pumping beam and the modulatedsignal beam, are involved in producing the electric polarization wave.It has been shown by P. K .Tien in the article Parametric Amplicationand Frequency Mixing in Propagating Circuits, Journal of AppliedPhysics, 29: 1347 (1958) and by I. A. Armstrong et al. in the articleInteractions Between Light Waves in a Nonlinear Dielectric, PhysicalReview 127:1918 (1962) that increased pumping power can overcome theeffects of small amounts of phase mismatch, such as occur inadvertentlybecause iof minor misalignments of the light beams or otherimperfections in a parametric amplifier in which provisions forphase-matching have been made.

I have recognized that this basic technique of compensating for phasemismatch is not limited to nonlinear interactions heretofore recognizedas phase-matchable and can be applied where the amount of phase mismatchis substantial, that is, Iorders of magnitude larger than contemplatedin the foregoing references. In certain crystals having largenonlinearities, it is only necessary to supply a suiiiciently largepumping power in relation to the other relevant parameters as describedhereinafter. Such large pumping power is possible with recentlydeveloped lasers, such as will be more fully specified hereinafter.

These lasers are sufficiently powerful to provide traveling waveparametric gain through the highly nonlinear non-phase-matchableinteractions in suitable materials with the simple adaptation ofintroducing the nonlinear material within the optical resonator of thelaser. Moreover, some of the recently ydeveloped pulsed lasers canprovide suiiicient power in a single mode that the crystal can bedisposed outside the resonant cavity.

It can be shown that the pumping power threshold for traveling waveparametric amplification with a substantial vector phase mismatch A isgiven by the relationship.

Threshold wDRoni Ai d wswiD ergs per second where ws is the angularfrequency of the signal beam, wp is the angular frequency of the pumpingbeam, wi is the angular frequency of the idler beam, Ap is thewavelength of the pumping beam in cm., Ro is the radius of the pumpingbeam in cm., c is the velocity of light in ctn/sec., np is the index ofrefraction of the material for the pumping wave, 11S is the index ofrefraction of the material for the signal wave, ni is the index ofrefraction of the material for the idler wave, and D in esu. is

the nonlinear second order polarization coemcient. A has the unitscnt-1. This threshold is the power required to yovercome the lossesproduced by vector phase mismatches .and is much greater than theconventional threshold for prior art parametric processes. The latterthreshold consists only of the power required to overcome absorption,reflection and defraction losses in the nonlinear crystal, which areusually negligible compared to the losses due to vector phasemismatches.

A single channel parametric amplifier .according to the invention isshown in FIG. 1. The amplifier comprises a crystal 11 of galliumarsenide, which is a noncentrosymmetric optically isotropic crystalhaving a large second order nonlinear coefcient, and having a thicknessof about one centimeter in the direction of the laser axis. A high powerpulsed uranium-doped calcium fluoride llaser 12, comprising the lasermedium 13, the exciting source 16 and the reflectors 17 and 18, suppliesa pumping beam directed at the crystal 11 through a focusing lens 9. Thecrystal 11 is provided with convex focusing surfaces 7 and 8 byconventional polishing techniques in order to provide the beam with aradius approximately equal to seven wavelengths of the pumping beamwithin the crystal 11. A signal source 14 supplies a modulated series ofpulses of radiation in the wavelength range between 3.25/t yand 13g.These pulses are synchronized with the pulsed exciting source 16 so thata series of signal pulses are applied to the nonlinear medium 11 at thesame time as it is being supplied with pulses from the Ilaser 12. Source14 directs the signal beam upon crystal 11 at a small angle,illustratively six degrees (6), yand with respect to the beam from laser12. A yreceiver 15 is positioned to receive the amplified signal beamfrom crystal 11.

Source 14 may be a transmitter. In the event that it is remote fromlaser 12, so that laser 12 and crystal 11 comprise a repeater, excitingsource 16 may be synchronized by appropriate control pulses receivedfrom signal source 14. Likewise, receiver 15 may be remote from laser 12and crystal 11. Together, the assembly of laser 12 and crystal 11 formsa communication repeater of the PCM type. With the signal source andreceiver, the entire assembly forms a communication system.

Pulsed laser 12, including active medium 13, near confocal reflectors 17`and 18 and the pulsed exciting source 16 is illustratively of the typedisclosed in the article Excitation, Relaxation yand Continuous MaserAction in the 2.613-Micron Transition of Ca F2: U3+, by G. D. Boyd etal., Physical Review Letters, Vol. 8, p. 269 (1962). The power level ofthe laser beam within the resonator is directly dependent upon the powerlevel of the exciting beam from source 16, which illustratively is amercury flash lamp. To obtain the desired power level, the laser source16 is operated on a pulsed basis.

It should be understood that reflectors 17 and 18 form the resonator forthe laser beam, although the exciting can also utilize focusingapparatus as disclosed in connection with FIG. 3 of the above-citedarticle.

Communication signal source 14 illustratively comprises a helium-xenonlaser operating upon an emission line at 5.5738u together with anelectro-optic shutter of known type adapted to modulate the laser beamin accordance with signal information on a pulsed basis. Thehelium-xenon laser, for example, is of the type disclosed in thecopending -application of W. R. Bennett, Jr., et al., Serial No.237,271, filed November 13, 1962 and assigned to the assignee hereof,and the electro-optic shutter illustratively comprises a nitrobenzeneKerr cell modulator disposed between crossed polarizers.

Communication receiver 1:5 is illustratively a photodetector of knowntype.

Gallium arsenide crystal 11 is oriented at an angle, with respect to theincident pumping beam, which produces -a nonlinear interactioneffectively utilizing its large second-order polarization coefficient.Illustratively, the

pumping radiation is incident perpendicular to one crystalline axis andat an oblique angle, i.e., 45", to the other two crystalline axes and ispolarized perpendicular or parallel to the one Iaxis. The polarizationof the signal radiation is not critical, so long as both the pumpingyand signal radiations are not polarized parallel to the samecrystalline axis; in other words, the pumping and signal radiationsshould have components that are polar- 4ized parallel, respectively, tomutually orthogonal crystalline axes. Inasmuch as crystal 11 isisotropic, the vector -b phase mismatch A, is independent of theorientation of the crystal. Further A depends only upon the anglebetween the pumping and signal beams, as would be expected fnomtrigon-ometric considerations, since phase vectors are involved.

While the embodiment of FIG. 1 involves pulsed ampliiication of thesignal, it is understood that continuous- Wave amplification could beachieved with a suitable continuous-wave laser and a continuous-wavesignal.` For example, for continuous-wave operation the laser could beof the type disclosed in connection with FIG. 4 hereinafter.

In the typical operation `of the embodiment of FIG. 1, laser 12 deliversto crystal 11 about 2960 watts of pulsed pumping power with a free spacewavelength of 2.6;@ and a beam radius of or 5.50u within crystal 11,where 3.31 is the index of refraction lof GaAs. It should beparticularly noted that the curved focusing surfaces '7 and 8 of crystal11 itself make feasible this relatively small beam radius of the pumpingbeam where it interacts with the signal beam. It is not easy to achievethe same effect with external focusing alone, i.e., by means of lens 9,because of diffraction limitations. Specifically, it isY extremelydiflicult to focus radiation to a beam radius between one and twowavelength with lens 9, because of diffraction limitations. It is notedthat the wavelength of the pumping radiation within crystal 11 is phasemismatch, Af?, of

the pumping power threshold for traveling wave interaction in accordancewith the invention is ideally about 740 watts, as calculated fromEquation l above. For the given 6 angle between the pumping and signalbeam,

A increases by a factor of about 1.8.

The interaction of pumping and signal beams in crystal 11 will generatean idler wave of a .frequency equal to the difference between pumpingand signal frequencies. Power will be transferred from the pumping beamto the signal and Iidler beams in a traveling wave interaction.

We can now calculate the total gain for one trave-rse through thecrystal 11. It is noted that, although crystal 11 is one centimeterthick to provide for focusing, the effective interaction length ld isapproximately the neareld distance, 100 kp in -this oase. For theexample being discussed, the effective interaction length, [(1:0.24

millimeter, approximately. The gain is calculated from therelationships;

Gain entl d where e is the base of the natural logarithms, ld is theinteraction distance, and

cnpnsni App-lied Pp=2960 watts and ld: M, aldilLS.

The total exponential parametric power gain for the collinear radiationsis e118=105o or 50' db.

For the six degree angle between the pumping and signal beams in theillustrative embodiment of FIG. 1,

{Al increases by a factor of 1.8, ld becomes slightly less than 1007\p,and the total exponential power gain is approximately 25 db. It shouldbe noted that with moderately increased pumping power, similarperformance can be achieved for larger angles between pumping and signalbeams.

A further condition for achieving this gain is that the input signalbeam be sufficiently low in power level that the pump power is notsubstantially depleted in providing the amplified output signal beams.At the specified gain level, this relationship is readily provided in acommunication system.

Obviously, this process can be repeated every few miles by repeaterscomprising components like laser 12, and crystal 11, with the repeatedsignal beam traversing the crystal at a small angle with respect to thelaser pumping beam as shown.

In lthe foregoing considerations, it was assumed that only parametricprocesses were taking place. In the presence of the large pumping powerat 2.6M, second harmonic generation at 1.3M will be a competing process.Nevertheless, this effect will be quite weak. No traveling wavebuild-upr of second harmonic occurs. It can be shown that the pumpingpower converted to second harmonic in a coherence length is less thanseven percent of the pumping power at 2.6M.

Moreover, the intensity of this harmonic power varies in an oscillatoryfashion along the length of the crystal 11 and lthe actual secondharmonic power loss in crystal 11 can be made very small by a slightrenement ofthe length of the propagation path for the pumping beam toterminate the interaction at a point where the second harmonic intensityis very small. The traveling wave amplifying properties of crystal 11for the signal beam would not be substantially altered by thisrefinement.

Inherent in the operation of the apparatus of FIG. 1 at high travelingwave gain levels is a broad bandwidth. Over the range of signalwavelengths from 'S25/t to 13p, the gain constant at varies by no morethan a factor of two. Actually lA] decreases as the signal and idlerdepart from the degenerate frequency of 52p; and thus the bandwidth iseven larger than just indicated. It is important to note that this istrue bandwidth, as opposed to the range of tunability, which is usuallylarge in parametric amplifiers, if mechanical adjustments are utilized.

Moreover, operation at high gain levels permits considerable tolerancein the angle between the pumping beam and signal beam. Ordinarily, themore nearly collinear the pumping beam and the signal beam, the higherthe gain. The variation is such that the six degree angle specifiedhereinbefore is a reasonable compromise between maximizing theefficiency and avoiding the complexity of the optics that would beneeded if the pumping and signal beams were to be made collinear.Moreover, angles greater than six degrees are feasible if higher pumpingpower is available. I consider angles up to ten degrees to be presentlyfeasible in View of available laser powers and interaction lengthconsiderations; and, as more powerful lasers become available, Iconsider this limiting angle to increase. Various modifications of theembodiment of FIG. 1 are possible.

The modified embodiment shown in FIG. 2, which is illustratively amultiple channel communication system, is significant in that thenon-phased-matched parametric amplification is achieved by pumping abirefringent nonlinear medium in a non-phase-matchable direction, i.e.,so that the signal and pumping beams must have the same polarization inorder to interact.

The crystal 21 is a noncentrosymmetric birefringent crystal of lithiumniobate (LiNbOa) oriented to have its optic axis perpendicular to theaxis of an argon-ion laser 22, which is the pumping source, and to haveits curved focusing surfaces 37 and 38 intercepting the laser axis. Thelaser comprises a tube 23 containing the argon active gas and having apumping or excitation source 26 and near confocal reflectors 27 and 28,the crystal 21 being disposed between tube 23 and reflector 28. Therefiectors 27 and 28 are adapted for and tuned to the 0.51/1 argonionlaser radiation. Signal sources 30 and 31 are disposed to project theirsignal beams upon crystal 21 through focusing surfaces 37 and 38- atsmall angles less than 1f) degrees with respect to the laser axis butsufficient to avoid the laser apparatus; and receivers 32 and 33 aredisposed, preferably remote from crystal 21, to receive thecorresponding amplified signal beams. It is noted that additional signalrepeaters may intercept the amplified signal beam before receivers 32and 33. In practice, the signal sources and receivers are disposed sothat the signal beams are arranged in a three-dimensional manner aroundthe pumping beam. As they will in general have different wavelengths,equalization of the gains for these signals is needed and can beachieved by making the angle between each of them and the pumping beaminversely related to the amount by which each departs from the so-calleddegenerate wavelengths, sd=12 tp=1-02u- For example, a 1.02# signal beamwould make a ten degree angle with the pumping beam; while beams orgreater or smaller wavelengths, within practical limits, would makesmaller angles with the pumping beam to equalize the gains for allsignals.

The radiations from the laser 12 and signal sources 30 and 31 areillustratively polarized parallel to the optic axis of crystal 21.

Argon-ion laser 12 is illustratively of the kind disclosed in thecopending application of E. I. Gordon et al., Serial No. 439,657, filedMarch 15, 1965 and assigned to the assignee hereof.

Crystal 21 is of the type disclosed in the copending application of A.A. Ballman et al., Serial No. 414,366, filed November 27, 1964 andassigned to the assignee hereof, with the modification that the curvedfocusing surfaces 37 and 38 are fashioned by conventional polishingtechniques to provide the desired small pumping beam radius withincrystal 21.

The operation of the system of FIG. 2 will be substantially the same `asthat of FIG. l for the following reasons. With the specified orientationof the lithium niobate crystal 21, the nonlinear interaction occurs bycoupling between the pumping and signal beams through the so-called D33second order nonlinear coefficient. This nonlinear interaction isconsidered to be non-phase-matchable because the pumping and signalbeams are polarized in the same direction in order to obtain theinteraction and because the crystal has appreciable normal dispersion inits index of refraction.

This mode of operation is to be contrasted with that disclosed forlithium niobate in the above-cited cepending application of A. A.Ballman et al. In that arrangement the lithium niobate crystal wasoriented to utilize its birefringence to obtain phase matching.

The D33 coefficient utilized in the arrangement of the present FIG. 2 isabout ten times larger than the D31 coefficient used in the arrangementof the above-cited Ballman application.

In operation, the pumping power threshold for collinear traveling waveexponential gain is of the order of 400 watts, inasmuch as the argon-ionlaser 22 is readily capable of operating at 0.51/1. with sufficientspectral purity to provide a beam radius Ro of approximately twowavelengths, 0.44/L, within the crystal 21. It is noted that crystal 21has an index of refraction of about 2.3. It may be seen from Equation 1that reduction of wavelength and beam radius has a marked effect inreducing the pumping power threshold. Moreover, continuouswave operationof the embodiment of FIG. 2 is possible because such a laser is capableof circulating powers within the resonator several times 400 watts on acontinuouswave basis.

The broad bandwidth of such an amplifier enables the simultaneousamplication of a multiplicity of communication signals.

The principles of the present invention are not limited to parametricamplification.

For example, it should be apparent that, in the embodiments of FIGS. 1and 2, if the signal beams are modulated, the idler beams are alsomodulated. Since power flows from the pumping beams to the idler beamsas well as to the signal beams, the arrangements of FIGS. l and 2 can beamplifying frequency converters. It is merely necessary to displace therespective receivers so that they receive the idler beams, which havewell-defined directions.

A second example of other uses is provided by the embodiment of FIG. 3,which limits the output power level of the beam which is nominally thepumping beam.

Crystal S1 is a crystal of gallium phosphide, oriented in the samemanner as described above for gallium arsenide in order to utilize itslarger nonlinear second order coefficient. The variable intensity lightsource 52 is a pulsed ruby laser of known type operating at 0.69/1 at avariable pulsed power level that is several times the threshold forgallium phosphide, as calculated from Equation 1 above. Le., laser 52supplies pulses of several tens of kilowatts intensity. Utilizationapparatus S4 is disposed to receive and utilize the unconsumed portionof the pulse from source 52, the power of which will be equal to thethreshold power.

No separate signal source is provided. However, background noise at somewavelengths longer than 0.69,:l will ordinarily be available; and, sincethe pumping power is so far in excess of the threshold, noise radiationshaving a variety of directions and frequencies will be amplified. Thesignificance of this noise amplification is that at the high gain levelprovided, the noise will substantially completely consume the pumpingpower in excess of the threshold.

Thus, the 0.69# beam received as a pulse by utilization apparatus 34will have a limited power level. In other words, the pulse is afiat-topped pulse. It should be understood that there are described inthe art a variety of uses for such a limited-amplitude pulse. An exampleis the precise promotion and control of a photo synthesis process, orother chemical reactions.

The advantage of a limiter as shown in FIG. 3 is that a clean-cutlimiting threshold is provided without resonant, or so-called high-Qcavities. The sharply defined 9 threshold is a direct consequence of thetraveling wave parametric amplification provided according to the prin--ciples of the invention.

As a point of interest, it should be noted that crystal 51 issubstantially opaque to the 0.35# second harmonic of the laserradiation; and there is no process strongly competing with the noiseamplification. It will be noted that second harmonics can be handled ineither of two Ways, as shown in the various embodiments; first, thecrystal can be transparent thereto and proportioned to generate verylittle thereof; second, the crystal can be substantially opaque thereto,the strong absorption inhibiting second harmonic generation. Anintermediate condition is ordinarily undesirable because the powerabsorbed will heat the crystal excessively.

The ability to amplify noise, as demonstrated by the embodiment of FIG.3 also makes apparatus according to the invention useful for producingoscillations.

A multiple frequency oscillator according to the invention is shown inFIG. 4.

Gallium arsenide crystal 61 is pumped by a neodyniumdoped yttriumaluminum garnet laser 62 -at 1.06,/i, as determined by the opaquereflectors 65 and 73 and pumping source 63. The 1.06p. pumping powercirculating between reflectors 65 and 73 through prism 64, lens 9 andcurved focusing surfaces 67 and 68 of crystal 61 is preferably severalkilo-watts of continuous-wave power. In crystal 61, various noisefrequencies compete for the gain; and those frequencies which areappropriately refracted by the dispersive glass prism 64 arepredominantly amplified because of multiple reflections between mirror65 and the partially transmissive mirrors 66-68.

Portions of these frequencies pass through the partially transmissiverefiectors 66-68 to be utilized in utilization circuits 69-71.Util-ization circuits 69-71 may be electrooptic modulators in which thepolarizations of the respective beams are modulated by linformationsignals to be received eventually by receivers of known type capable ofanalyzing modulated polarization. Amplified noise that is notappropriately retracted passes out of the system without interferingwith the desired outputs.

ln order to provide the appropriately large pumping power substantiallyexceeding the threshold calculated from Equation 1 above, neodyniumlaser 62 may illustratively be of the type disclosed inthe copendingapplication of I. E. Geusi-c et al., Serial No. 367,306 filed May 25,1964.

Crystal 61 could be replaced by a gallium phosphide crystal, which islikewise n-oncentrosymmetric and isotropic but requires a greaterpumping power than gallium arsenide. Gallium phosphide is relativelytransparent to the 0.53,a second harmonic that is generated within thecrystal 61 by the 1.06# pumping beam.

In all cases it is understood that the above-described arrangements areillustrative of a small number cf the many possible specific embodimentsthat can represent applications of the principles of the invention.Numerous and varied other arrangements can readily be devised inaccordance with these principles by those skilled in the art Withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. A traveling wave parametric device comprising a body of anoncentrosymrnetric material having a substantial nonlinear second orderpolarization coefficient that describes a non-phase-matchableinteraction that couples incident waves to produce an idler wave, andmeans for providing traveling wave exponential parametric amplificationfor a signal beam, comprising means for applying said signal beam and apumping beam to said material in directions to be coupled through saidnon-phase-matchable interaction, the angle between the direct-ions ofsaid signal and pumping beams being less than ten degrees.

2. A device according to claim 1 in which the material is an Iopticallyisotropic crystal.

3. A device according to claim 1 in which the material is a birefringentcrystal having a nonlinear coefficient that describes an interactionthat is not phase-matchable through birefringence and the applying meansdirects the pumping and signal beams to be coupled .through saidinteraction.

4. A device according to claim 1 in which the material is a crystal forwhich the strongest possible nonlinear interaction per unit interaction`length is the non-phasematchable interaction.

5. A device according to claim 1 in which the material is a crystalselected from the group consisting of gallium arsenide, galliumphosphide and lithium niobalte.

6. A traveling Wave light amplifier comprising a body oflight-transmitting noncentrosymmetric material capable of couplingpumping, signal and idler waves through` a nonlinear interaction that isnon-phase matchable, said interaction being design-ated by a secondorder nonlinear coefiicient D in esu., said body having opposed curvedfocusing surfaces; means for applying a signal beam of angular frequencyws to said material; and means for providing traveling wave exponentialparametric amplification for said signal beam by applying a pumping beamof angular frequency wp to said material in a direction that interceptssaid curved focusing surfaces to couple energy to said signal beam andproduce an idler wave of angdilar frequency w1 through said nonlinearinteraction with a substantial vector .phase mismatch A in cml, saidpumping beam having a Wavelength kp in cm., a beam radium Ro in cm., anda power level greater than where c is the velocity of light in cm./sec.,np is the index of refraction of the material for the pumping beam, nsis the index of refraction of the material for the signal beam, and n1is the index of refraction of the material for the idler wave, the anglebetween the directions of said pumping and signal beams being less thanten degrees.

7. A traveling wave light `amplifier according to claim 6 in which thematerial is a crystal of gallium arsenide oriented to have onecrystalline axis orthogonal to the direction of the pumping beam and twoother crystalline axes oblique to the direction of the pumping beam.

S. A traveling Wave light amplifier according to claim 6 in which thematerial is a crystal of lithium niobate oriented to have its optic axisorthogonal to the direction of the pumping beam.

9. A traveling wave light amplifier according tto claim 6 in which thepumping beam applying means is a laser having a resonator and the bodyof material is disposed within said resonator.

10. Parametric wave translation apparatus comprising a crystal of anonlinear isotropic optical medium, means for passing a signal beamthrough said crystal, and means for providing traveling wave exponentialparametric ampliiication for said signal beam, comprising a pulsed solidstate laser adapted to provide a pumping beam with a power level,direction and radius appropriate for said traveling wave parametricamplification in said crystal, the angle between the direct-ions of saidsignal beam and said pumping beam being less than =ten degrees.

11. An optical multiple frequency oscillator comprising a crystalcapable of a nonlinear optical interaction for which substantial vectorphase mismatching is obtained for any direction of pumping and signalbeams that will produce said interaction, said crystal having opposedcurved focusing surfaces intercepting one such direction, meansincluding a dispersive element and a plurality of partially transmissivereiiectors disposed around said crystal and said dispersive element toform individual resonators for a corresponding plurality of noisefrequencies, and means for providing traveling wave exponential para- 11 metric amplication of said plurality of noise frequencies through saidnonlinear optical interaction, including means for directing a pumpingradiation beam with appropriate power, radius and direction through saidcurved focusing surfaces.

References Cited by the Examiner UNITED STATES PATENTS 12 ReferencesCited by the Applicant Physical Review, vol. 127, 1962, article by I. A.Armstrong et al., page 1918.

Applied Optics, vol. 1, 1962, article by A. E. Siegman, page 739.

Journal of Applied Physics, vol. 29, 1958, article by P. K. Tien, page1347.

ROY LAKE, Primary Examiner.

3,200,342 10/1965 Kibler 33o-4.3 10 D. R. HOSTETTER, Assistant Examiner.

1. A TRAVELLING WAVE PARAMETRIC DEVICE COMPRISING A BODY OF ANONCENTROSYMMETRIC MATERIAL HAVING A SUBSTANTIAL NONLINEAR SECOND ORDERPOLARIZATION COEFFICIENT THAT DESCRIBES A NON-PHASE-MATCHABLEINTERACTION THAT COUPLES INCIDENT WAVES TO PRODUCE AN IDLER WAVE, ANDMEANS FOR PROVIDING TRAVELING WAVE EXPONENTIAL PARAMETRIC AMPLIFICATIONFOR A SIGNAL BEAM, COMPRISING MEANS FOR APPLYING SAID SIGNAL BEAM AND APUMPING BEAM TO SAID MATERIAL